1. Field of the Invention
This invention relates to optical recording/reproducing apparatus such as an optical disc unit which uses optical discs for recording or reproducing.
2. Description of the Prior Art
An optical disc unit records or reproduces information optically by directing a laser beam onto an optical disc. Recently, optical disc units are finding many useful applications such as large-capacity information filing.
An optical disc is made with groove tracks, called "pregrooves", formed spirally or concentrically on the disc, and a recording beam forms pits in the pregrooves to record data. To reproduce, a beam is applied to the grooves and the pits detected from the difference in the amount of light reflected after passage through the pits. The reproducing beam and the higher energy recording beam are derived from a single laser beam. A problem exists, however, that the reproducing beam cannot be reflected from the recording pits unless the diameter of each recording pit is greater than the wavelength of the reproducing beam. If the recording beam has the same wavelength as the reproducing beam, reflection from the recording pit and correct reproduction are difficult.